How to Repair a Sewer Pipe Under a Concrete Slab
Break through an interior concrete slab, excavate to access a broken sewer line, replace the damaged section with PVC, and seal the opening.
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🧰 Required Tools & Equipment
- ✓ Rotary hammer or demo hammer
- ✓ Reciprocating saw with carbide metal blade
- ✓ Torque wrench or 5/16-inch preset torque driver
- ✓ Trenching shovel
- ✓ Wire brush
- ✓ Tape measure
- ✓ Concrete finishing trowel
📦 Materials & Supplies
- ✓ Schedule 40 PVC sewer pipe
- ✓ Shielded rubber transition couplings (Mission/Fernco)
- ✓ Crushed stone or pea gravel bedding
- ✓ Concrete mix (high-strength)
- ✓ 6-mil poly vapor barrier plastic
- ✓ PVC primer and cement
⚠️ Safety Checkpoints & Warnings
- • Wear an N95 or P100 respirator and eye protection whenever breaking concrete to prevent silica dust inhalation.
- • Verify no submerged electrical conduit or gas lines run in the trench path before digging.
- • Keep workspace actively ventilated to prevent sewer gas buildup from open pipe ends.
📋 Step-by-Step Guide (6 Steps)
Break slab and excavate around the pipe
Chisel a rectangular opening through the concrete slab directly above the leak zone with at least 12 inches of clearance on all sides. Dig out soil and gravel around the pipe, clearing a 6-inch pocket beneath the bottom of the pipe so couplings can wrap around freely.
- • Concrete slab cleared back at least 12 inches on all sides of the break
- • A 6-inch open gap visible directly underneath the entire circumference of the pipe
- • Digging too tight of a trench, preventing wrenches and hands from wrapping around pipe bottom.
Clean and cut out the damaged pipe section
Scrape rust, scale, and dirt from the exterior of the existing pipe where the couplings will mount using a stiff wire brush. Use a reciprocating saw with a carbide-tipped blade to make square, perpendicular cuts at least 3 inches past the cracked or rusted zone.
- • Damaged section completely removed leaving a clean square cut on both remaining pipe stubs
- • Pipe exterior scrubbed down to bare, smooth metal or plastic with no scale ridges
- • Cutting at an angle, causing gaps inside the replacement sleeve that trap waste.
Measure and cut replacement PVC spool
Measure the exact gap between existing pipe ends and cut a piece of Schedule 40 PVC to match, subtracting 1/4 inch total to allow clearance for rubber coupling stops. Deburr the inside and outside edges of the cut PVC with a file or utility knife.
- • Replacement PVC segment matches gap length within 1/8 inch
- • Chamfered, smooth PVC cut ends free of plastic burrs or shavings
- • Cutting the PVC piece too long, forcing the existing pipe out of pitch when wedged in.
Install shielded transition couplings
Slide the rubber sleeves and outer stainless-steel shields onto the existing pipe stubs, folding the rubber edges back if necessary. Position the PVC repair pipe in place, fold rubber sleeves over the joints, slide stainless shields over the rubber, and tighten the band clamps with a torque driver to 60 in-lbs.
- • Metal shields centered precisely over both rubber joints
- • Band clamp screws tightened evenly until the metal band draws completely flush without wrinkling
- • Using unshielded all-rubber Fernco couplings underground, which can deflect and create internal offsets.
Test flow and bed pipe with aggregate
Run water from an upstream fixture to check the joints for any leaks or weeping around the bands. Shovel crushed stone or pea gravel underneath and around the pipe, tamping it firmly with a 2x4 to create a solid, non-settling bed up to the pipe midline.
- • Coupling joints remain bone-dry during continuous water flow test
- • Gravel packed tightly around bottom half of pipe with no hollow voids beneath
- • Backfilling directly with native soil or large rocks, which will settle and snap or misalign the pipe.
Backfill, place vapor barrier, and pour concrete patch
Backfill with compacted gravel up to the underside of the concrete slab and lay a 6-mil poly vapor barrier sheet overlapping the trench edges. Mix concrete, shovel into the cutout until slightly crowned, and strike off flush with the surrounding floor using a trowel.
- • Plastic vapor barrier tucked under slab perimeter edge
- • Fresh concrete troweled flat and seamless with existing floor slab surface
- • Omitting the vapor barrier, causing future moisture transmission and floor covering delamination.
Credit: inspired by an original tutorial by This Old House — watch the original on YouTube.
Don’t have everything?
- No Demo hammer? Sledgehammer and cold chisel (slower and higher physical fatigue)
- No Preset 60 in-lb torque driver? Standard 5/16-inch nut driver tightened firmly until rubber bulges slightly against metal shield bands